In a digital communications system, digital audio data and control information is transmitted in a predetermined serial transmission format such as AES-EBU (Audio Engineering Society-European Broadcast Union) or CP-340 (a name given the standard developed by the Electronics Industries Association of Japan). Both the AES-EBU and CP-340 formats were developed for serial transmission of two channels, each having digital audio data and non-audio, or control, data from a transmitter to one or a plurality of receivers. For more detailed information on the AES-EBU format, refer to "AES Recommended Practice for Digital Audio Engineering-Serial Transmission Format for Linearly Represented Digital Audio Data" published by the Audio Engineering Society in 1985. Similarly, for information concerning the CP-340 format, refer to "EIAJ CP-340 Digital Audio Interface" published by the Standards of Electronic Industries Association of Japan in 1987.
Both the AES-EBU and CP-340 formats are commonly used for transmitting digital audio and non-audio data between a compact disc player, a digital audio tape player, an audio mixing board, studio recording equipment, and consumer musical instruments. Because of the wide applications of the AES-EBU and CP-340 formats for transmission of audio information, it is useful for a digital signal processor to also be compatible with this digital audio format. When transferring digital audio information from a transmitter, such as a compact disc player or a digital audio tape player, to a digital signal processor, the digital data is typically provided to an interface transceiver where it is modified to a form in which it may be used by the digital signal processor.
In some cases, several sources of digital audio data may be used to provide signals which are related. For example, in a recording studio, an audio mixing board may be used in conjunction with several digital audio storage devices. Each of the digital audio storage devices has an input which is compatible with the AES-EBU or the CP-E40 format. These storage devices may be hard disk drives, digital audio tape players, or even compact disc players with standard pre-recorded samples.
Several techniques exist for configuring a system which has multiple sources of digital audio data. For example, a digital signal processor (DSP) may be dedicated for use with each one of the interface transceivers. With appropriate synchronization, the digital audio data would then be processed in parallel by the digital signal processors. However, digital signal processors are typically costly. Therefore, the number of digital signal processors implemented in a system should be minimized to decrease expense.
In some telecommunications applications, multiple digital data sources are interfaced with a microprocessor using a time slot assigner circuit. In such a system, the digital data sources are typically codecs. An example of a time slot assigner circuit is a Motorola MC14416 circuit which is commercially available from Motorola, Inc. of Schaumburg, Ill. In a system which uses a time slot assigner circuit, each of the multiple digital data sources is assigned a time slot for communication with the microprocessor. The time slot assigner enables a first one of the plurality of digital data sources to communicate with the microprocessor at a first time slot. At a second time slot, the time slot assigner enables a second one of the plurality of digital data sources to communicate with the microprocessor. In summary, each one of a plurality of digital data sources is assigned a time slot for communication with the microprocessor. Additionally, the added circuitry is costly to implement in a communications system and should be eliminated whenever possible.
Similarly, a digital signal processor may interface with a plurality of digital data sources. The plurality of digital data sources are generally implemented as either other digital signal processors or codecs. Like the time slot assigner circuits, this implementation is provided only for telecommunications applications and digital audio communication applications. To interface with the plurality of digital data sources, the digital signal processor must be operated in a specific mode of operation which is generally referred to as "Network Mode." In such a mode of operation, the digital signal processor would be a "master" device which controlled all of the plurality of digital data sources. The digital signal processor assigns each one of the plurality of digital data sources a time slot in which data is to be either transmitted or received. Additionally, the digital data sources must be able to recognize which time slot is currently executed and whether data should be communicated. Therefore, such a system requires "smart" digital data sources which are able to determine and track a time slot. Such smart digital data sources are usually digital signal processors. Other commonly used digital audio devices generally do not have the "intelligence" to recognize time slots without some sort of human support. Again, however, digital signal processors are costly to implement and should be minimally used in a system. The network mode of operation is implemented on the Motorola DSP 56000 digital signal processor which is available from Motorola, Inc. of Austin, Tex. For more information on the network mode of operation and the DSP56000, refer to "DSP56000/DSP56001 Digital Signal Processor User's Manual."
The telecommunications applications described herein do not correlate well or provide a solution to a problem which is increasingly occurring in digital audio data processing systems. Multiple digital sources are required to provide a plurality of digital audio signals. However, multiple digital signal processors which are required to process each of the plurality of digital audio signals are typically too costly to implement. The telecommunications applications described herein are too costly to implement in a digital audio communications system and do not provide the flexibility required by such a system.
Therefore, a need exists for a digital audio communications system in which a minimal number of digital signal processors may be used with a plurality of digital audio sources to provide digital audio information without great expense.